Image-forming MR methods which utilize the interaction between magnetic fields and nuclear spins in order to form two-dimensional or three-dimensional images are widely used nowadays, notably in the field of medical diagnostics, because for the imaging of soft tissue they are superior to other imaging methods in many respects, do not require ionizing radiation and are usually not invasive.
According to the MR method in general, the body of the patient to be examined is arranged in a strong, uniform magnetic field whose direction at the same time defines an axis (normally the z-axis) of the co-ordinate system on which the measurement is based. The magnetic field produces different energy levels for the individual nuclear spins in dependence on the magnetic field strength which can be excited (spin resonance) by application of a pulsed electromagnetic alternating field (RF pulse) of defined frequency (so-called Larmor frequency, or MR frequency). After termination of the RF pulse, a MR signal can be detected by means of a receiving RF antenna (also referred to as receiving coil) which is arranged and oriented within an examination volume of the MR device in such a manner that a temporal variation of the net magnetization of the body of the patient is measured in the direction perpendicular to the z-axis. In order to realize spatial resolution in the body, linear magnetic field gradients extending along the three main axes are superposed on the uniform magnetic field, leading to a linear spatial dependency of the spin resonance frequency. The MR signal picked up by means of the receiving RF antenna then contains components of different frequencies which can be associated with different locations in the body.
Typically, the level of the electromagnetic alternating field during the RF pulse is orders of magnitude larger than the MR signal generated by the excited nuclear spins and detected by the RF receiving antenna. To obtain a maximum signal to noise ratio (SNR) the receiving RF antenna is typically part of a RF resonant circuit configured to resonate at the MR frequency. To maintain safety and to protect the sensitive RF receiving equipment including the reception antenna and the resonant circuit, the RF resonant circuit is usually detuned while RF pulses are irradiated. Known RF reception hardware therefore comprises a switching circuitry which is configured to switch the RF resonance circuit between a resonant mode and a non-resonant mode. MR signal acquisition takes place in the sensitive resonant mode, i.e. during the receive phase of the imaging procedure, while the RF resonant circuit is switched to the non-resonant mode during the transmit phase. In the non-resonant mode the resonance frequency of the RF resonant circuit is shifted away from the MR frequency. In this way, the dangerous induction of high voltages in the RF resonant circuit during the transmit phase is effectively avoided.
Accordingly, it is known to detune the reception circuitry in a MR system by using semiconductor switches or PIN diodes in connection with appropriate LC circuitry. Two principal variants are commonly used (see for example WO 2008/078270 A1), namely active detuning and passive detuning.
With active detuning, a bias voltage is applied to a PIN diode semiconductor switch in conjunction with an LC circuit to detune the RF reception coil during the transmit phase of the imaging procedure. A disadvantage of the active detuning approach is that an external switching signal is needed for switching the RF resonant circuit between the resonant mode and the non-resonant mode. This increases the complexity of the MR imaging system. A further drawback is that, due to the high power of the RF pulses, a correspondingly high bias voltage needs to be applied to the switching diodes to ensure that the receiving circuitry remains decoupled during RF irradiation. This high bias voltage increases design complexity and heat dissipation in the corresponding DC supply lines. Moreover, the current resulting from the high bias voltage induces field distortions in the main magnetic field, thereby degrading image quality.
With passive detuning, anti parallel diode semiconductor switches are used in conjunction with LC circuitry. In this approach, anti parallel combinations of high speed switching diodes detune the RF resonant circuit in response to the RF pulse itself. In other words, when the anti parallel combination of diodes is exposed to the high power signal of the RF pulse, each diode conducts during its respective half cycle of the RF radiation. A major drawback of the passive detuning is that in the case of low flip-angle RF pulses the self-biasing effect of the anti parallel diodes is too small. Consequently, no reliable detuning of the RF resonant circuit during the transmit phase is achieved.